Teaming up main group metals with metallic iron to boost hydrogenation catalysis

Hydrogenation of unsaturated bonds is a key step in both the fine and petrochemical industries. Homogeneous and heterogeneous catalysts are historically based on noble group 9 and 10 metals. Increasing awareness of sustainability drives the replacement of costly, and often harmful, precious metals by abundant 3d-metals or even main group metals. Although not as efficient as noble transition metals, metallic barium was recently found to be a versatile hydrogenation catalyst. Here we show that addition of finely divided Fe0, which itself is a poor hydrogenation catalyst, boosts activities of Ba0 by several orders of magnitude, enabling rapid hydrogenation of alkynes, imines, challenging multi-substituted alkenes and non-activated arenes. Metallic Fe0 also boosts the activity of soluble early main group metal hydride catalysts, or precursors thereto. This synergy originates from cooperativity between a homogeneous, highly reactive, polar main group metal hydride complex and a heterogeneous Fe0 surface that is responsible for substrate activation.

Chemical shifts (δ) are denoted in ppm (parts per million) and coupling constants in Hz (Hertz). 1 H and 13 C NMR spectra were referenced to the solvent residual signal (SiMe4 = 0 ppm). Signal multiplicities are described using common abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), quint (quintet), m (multiplet) and br (broad). CHN Elemental analysis was performed with a Hekatech Eurovector EA3000 analyzer or commissioned to an external company: Mikroanalytisches Laboratorium Kolbe, Oberhausen, S3 Germany. Quantitative CHN and metal analyses of the Fe 0 and Ba 0 samples obtained by cocondensation was commissioned to Mikroanalytisches Laboratorium Kolbe. GC/MS measurements were performed with a Thermo Scientific™ Trace™ 1310 gas chromatography system (carrier gas helium) with detection by a Thermo Scientic™ ISQ™ LT Single Quadrupole mass spectrometer. A Phenomenex® ZebronTMZB-5 column of the dimensions 0.25mm x 30m with a film thickness of 0.25 μm was used. The samples (1 μl) were injected with an Instant Connect-SSL Module in the split mode (Injector Temperature: 280 °C, split ratio 0.9, carrier gas flow 1.2 mL/min). Conditions for mass spectrometry: ion source temperature 280 °C, ionizing energy (70 eV), mass range 20-500 (m/z). The molecular identity was confirmed by comparison with entries in the NIST/EPA/NIH mass spectral library (version 2.2, built June 10 2014).
The particle morphologies of powdered Fe 0 , Ba 0 and a spent FeBa catalyst were analyzed by scanning electron microscopy (SEM) using an Apreo S Lovac microscope and by transmission electron microscopy (TEM) studies using an Jeol JEM 2200 fs microscope equipped with probe-side Cs-corrector operated at 200 kV acceleration voltage. Samples were inserted with an Air-Free Transfer TEM holder. X-ray diffraction (XRD) patterns were collected with a Stoe STADI P diffractometer with Mo Kα1 radiation (λ: 0.7093 Å, 40 kV, 40 mA). The powdered catalysts were measured in sealed capillaries (Ø 500 µm) to prevent oxidation. X-ray photoelectron spectroscopy (XPS) was performed using a Versaprobe II™ from Ulvac-Phi with monochromatic Al Kα light at 1486.6 eV photon energy and an 45° emission angle between analyser and sample.
All hydrogenation experiments were carried out in stainless steel high-pressure autoclaves (volume: 15 mL) made by Advanced Machinery and Technology Chemnitz GmbH (Amtech). Autoclaves and stirring bars have never been in contact with transition metal catalysts. Autoclaves were only cleaned with dilute solutions of acetic acid to avoid metal abrasion and were dried prior to use by heating in an oven at 80 °C overnight. The reactors were connected by flexible high pressure tubing to a high pressure device consisting of a pressure regulator (1-50 bars) and a metering valve and pressurized with hydrogen (H2, Air Liquide, 5N-purity: 99.999%).

General considerations:
CAUTION: All activated metal powders are extremely pyrophoric and burn vigorously in air. S4 The metals and a volatile organic matrix were evaporated under high vacuum and cocondensed together on a glass surface which was cooled to the temperature of liquid nitrogen. The home-build reactor follows the principle of a common oil diffusion pump with backing pump and cold trap operating under high vacuum in the range of 10 -4 to 10 -6 mbar. [2,3] The heating unit encloses an aluminum oxide container for the metal to be evaporated. The organic matrix, which cocondensed with the metal, was introduced through manifolds. In a typical Metal Vapor Synthesis (MVS) experiment, the metal (ca. 1.5-5.0 g) was cut into pieces of circa 5 mm which were placed in the aluminum oxide crucible and the reactor was evacuated to ca. 10 -5 mbar. The crucible containing the metal was heated up to ca. 250 °C for several hours. During this process all volatiles, including humidity on the glass surface, were removed and collected in the cold trap. After previous cooling of the apparatus to room temperature, the reaction flask was cooled further with liquid nitrogen. Subsequently, ca. 10 mL of organic matrix was introduced through the manifold by evaporation. The matrix condensed on the cold glass surface and formed a thin layer of frozen organic solvent on the glass wall. This organic layer acts as a spacer between the flask and cocondensed metal and assists in draining and isolation of the metallic slurry. Subsequently, the metal in the aluminum oxide crucible was heated to the temperature needed for metal vaporization while the organic matrix (ca. 140 mL) was continuously introduced. The desired onset temperature was reached when metal condensation on the glass walls started, as indicated by formation of vividly colored cocondensates of finely divided metal. In case of alkaline-earth metals, often blue or yellow/green colors appeared which later turned black or brown, depending on the solvent. At the onset of metal evaporation, the crucible temperature was measured by an internal heat sensor. The onset temperature is generally equal to the melting point of the metal. To maintain a significant metal vapor flow, the temperature during the cocondensation experiment was adjusted circa 100 °C higher than the onset temperature. Typical values are dependent on the machine and the quality of the vacuum but a guideline for the operation temperature can be found below for the individual metals. The cocondensation process was stopped after all solvent (matrix) had been cocondensed into the reaction flask (ca. 1 h). After completion of metal cocondensation, the cooling bath under the condensation flask was removed, the reactor was filled with nitrogen gas and was allowed to warm up to room temperature. The metal suspension that drains to the bottom of the flask can be easily isolated by transferring it into a syringe with a long stainless steel cannula. The metal suspension was collected in a centrifuge Schlenk tube and after centrifugation and removing of the supernatant, the remaining metal powder was dried in vacuum at room temperature and stored in a glovebox. Generally, ca. 70% of the initially introduced metal could be isolated in the form of highly pyrophoric activated metal powder.

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Activated alkaline-earth metals Ba, Sr, Ca and Mg: An aluminium oxide crucible was completely filled with alkaline earth metal pieces. Typical amounts used: Ba ca. 4.0 g, Sr ca. 2.0 g, Ca ca. 1.5 g and Mg ca.
2.0 g. MVS was carried out as described above at a pressure of circa 10 -5 mbar and at temperatures that are circa 100 °C higher than the metal melting points; melting points: Mg 650 °C, Ca 842 °C, Sr 777 °C, Ba 727°C. For the metals Ba, Sr and Ca, n-heptane was used as the organic matrix. Cocondensation of Mg and n-heptane gave larger unreactive Mg lumps, however, using THF as a matrix led to a highly reactive Mg powder. Generally, ca. 70% of the initially introduced metal could be isolated in the form of highly pyrophoric activated metal powder. The activated Ba metal powder that was obtained by this method was analyzed for metal content. Elemental analysis (w%): Ba 93.72, C 1.38, H 0.34, N 3.24. The high N value is due to adsorption of N2 on the Ba 0 surface.

Activated iron:
Iron was activated by cocondensation with an organic solvent at a vacuum of 10 -5 mbar and an operation temperature that is circa 100 °C higher than the melting point of Fe (mp: 1535 °C). When n-heptane was used, the obtained iron powder formed an agglomerate which could not be removed by cannula transfer from the reaction vessel, presumably due to the magnetic nature of the material. However, using toluene during the cocondensation process a red solution of Fe(toluene)2 was obtained. This labile complex [4] decomposed above ca. -60°C to a fine slurry of metallic Fe 0 which after standing for two days separated from the organic fraction.
Although not strictly necessarily, it was found that addition of hexamethyldisilazane, HN(SiMe3)2, during the cocondensation process gave an increased yield of activated Fe 0 . Optimization led to the following method: Iron chunks (ca. 5.0 g) were cocondensed in a manner as described above for the alkaline-earth metals but the organic matrix was replaced by toluene (150 mL) and hexamethyldisilazane (HN(SiMe3)2, 20 mL) and at a vacuum of 10 -5 mbar the operation temperature was circa 1635 °C (be aware that this is machine and vacuum dependent). The metal powder was separated from the mother liquor by decantation, washed several times with 10 mL portions of toluene and after isolation dried in vacuum at room temperature. The activated Fe powder was stored under N2 in a glove box at room temperature.

Solid state investigation on Ba 0 , Fe 0 and the ground BaFe mixture
During our investigations on alkene hydrogenation with Ba 0 /Fe 0 mixtures, it was found that grinding the two metal powders with mortar and pestle is advantageous for the catalytic activity. Activated Ba 0 , Fe 0 and the ground BaFe mixture have been characterized by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The quality of the TEM measurements was negatively affected by problems to disperse the samples using sonication. The SEM and XPS measurements were affected by problems to insert the samples in the machine under inert conditions. The p-XRD data showed that the Fe 0 , Ba 0 and BaFe catalysts are crystalline nanosized metallic powders with particle diameters of circa 5 nm. According to XPS measurements, the surfaces not only consist of metallic Fe 0 and Ba 0 but are partially oxidized which is related to their highly reactive pyrophoric nature.
The metal samples were stored in closed ampoules under high vacuum and the XPS samples were freshly prepared in a glovebox under inert gas conditions (Ar, < 0.01 ppm O2). However, the reactivity of these nanostructured catalysts is sufficiently high that their surfaces already decomposed by traces of O2 and H2O during sample insertion. The color of these partial decomposed samples remained pitch-black indicating that only the surface was partially oxidized due to sample insertion (Ba 0 should become white upon oxidation whereas Fe 0 would turn brown). Figure S1 shows the powder X-ray diffractograms (p-XRD) for the Ba 0 , Fe 0 and the ground BaFe catalyst.
These samples were measured in sealed capillaries which were filled in the inert atmosphere of a glovebox (Ar, < 0.01 ppm O2) and therefore these spectra are not affected by partial surface oxidation. Only Bragg reflections corresponding to the metallic phases of Ba 0 and Fe 0 are visible, while reflexes due to the presence of other crystalline phases including metal oxides or hydroxides, were not observed. Although metallic barium usually crystallizes in a body-centered cubic lattice ( -Ba), the observed p-XRD pattern is in good agreement with the reported pattern for its β-modification which crystallizes in a face-centered cubic lattice. The β-modification of Ba 0 was first observed for barium particles obtained by metal evaporation and spraying into high vacuum, [5] i.e. conditions similar to those used for the preparation of the activated Ba 0 catalyst. Based on the peak width, the particle size was calculated to ca. 5 nm for both the Ba 0 and the Fe 0 catalysts. SEM photographs of the Fe 0 and Ba 0 catalysts are displayed in Fig. S2. These show that the nanoparticles have a high tendency to agglomerate, resulting in the formation of large particle clusters with roughly 100 nm in size. These clusters are composed of much smaller, uniform particles. The acquisition of high-S7 resolution images is made difficult caused by the partial oxidation of the extremely reactive particles as a result of the short exposure to traces of air during transfer into the SEM sample chamber.  TEM photographs of Fe 0 , Ba 0 and a spent BaFe catalyst are displayed in Fig. S3. As the samples were inserted in the machine using an Air-Free Transfer TEM holder, surface oxidation was only minimal. The S8 TEM photographs show the formation of large particle agglomerates which consist of very small, uniform sub-10 nm particles. The high agglomeration tendency is due to the specific synthetic method which avoids use of sterically demanding organic molecules (capping agents) on the particle surface and complicates the acquisition of high-resolution images. Further dispersion of these particles by sonication was not possible. For further insight into the elemental composition and the valence states of the elements on the surface, X-ray photoelectron spectroscopy (XPS) was performed in the SEM. The XP survey spectra shown in Fig.   S4 confirm the presence of the respective metals in addition to carbon and oxygen for the Ba 0 and Fe 0 catalysts, respectively. The Ba 0 catalyst also shows adsorbed N2 which is typical for this metal. The XP spectrum for the ground BaFe catalyst is merely a superposition of Ba 0 and Fe 0 spectra. This is in agreement with the observation that Ba and Fe do not form alloys. [6] S9 Figure S4. Survey XPS for the activated Ba 0 , Fe 0 and BaFe catalysts.   S8) can be deconvoluted into two peaks, whereas the main peak at 528.5 eV corresponds to oxygen in metal oxide and the minor peak at 531.6 eV to barium hydroxide. [8] S11 Figure S7. High-resolution XP spectrum of the Ba 3d region for the Ba 0 catalyst. Figure S8. High-resolution XP spectrum of the O 1s region for the Ba 0 catalyst. S12 The spatial distribution of the elements Ba and Fe within the BaFe catalyst powder was determined by EDX mapping. Figure S9 shows the SEM photograph of a larger area of the native powder and the corresponding elemental mappings. Since the sample could not be introduced in the machine under purely inert conditions, partial oxidation/hydrolysis of the surface is to be expected but this does not influence the Ba/Fe distribution. It can be clearly seen that the elements barium (red) and iron (green) are spatially separated and present in micrometer sized areas. This distribution is due to the production method in which activated powders of pure Ba and Fe are mixed with a mortar and pestle. The composite image shows that the Ba-rich and the Fe-rich regions are complementary to each other.
The elemental composition of the catalyst in the native state was determined by EDX measurement (Fig.   S10). This technique determines the nature and quantity of elements at the surface of the material (up to a depth of circa 1 m). Due to partial oxidation of the surface, not only Ba and Fe but also O is present but this does not affect the Ba/Fe ratio. As the distribution of Ba and Fe in the native BaFe catalyst is rather inhomogeneous, the Ba/Fe ratio was determined by EDX for two larger areas of circa 200 x 200 µm (Table   S1). The results of the two measurements are in good agreement and show an equimolar Ba/Fe ratio which corresponds to the 1/1 Ba/Fe ratio used for its preparation.
Also the spent catalyst (after hydrogenation of benzene at 150 °C and 50 bar H2 pressure) was analysed by EDX mapping. Figure S11 shows that there is no significant change in the spatial distribution of Ba and Fe. It is, however, clear that the surface of the catalyst after use is much richer in Ba than in Fe. This was confirmed by EDX mapping in four different areas of circa 200 x 200 µm (Table S2). Although this showed a strongly heterogeneous Ba/Fe distribution, it is clear that the surface of the catalyst is enriched in Ba.
Instead of the expected Ba/Fe ratio of 1/1 for the native catalyst, a Ba/Fe ratio of circa 2/1 is observed.
Since the catalyst is fully recovered from the reaction mixture (see Figure S16   This is the volume needed to fill a 5 mm NMR tube and the total reactor contents were, after filtering in order to remove paramagnetic Fe, transferred to an NMR tube for direct no-D NMR measurement (measurements without deuterated solvents and lock; internal spectrometer frequency was used for referencing). Solid substrates were first dissolved in n-heptane and added to the reactor as a solution.
After loading the reactors in a glovebox (N2, <0.01 ppm O2), they were and transferred outside to be pressurized with hydrogen to the specified pressure. After closing the gas inlet, the autoclaves were then placed into preheated aluminum blocks on magnetic stirrers and quickly reached the desired temperature. After a given time, the reactor was cooled to room temperature and conversion was determined by means of 1  The catalytic activity of the BaFe catalyst is compared to earlier reported results on the catalytic activity of Ba 0 . [9] Since TOF values are instantaneous and strongly dependent on time and substrate conversion, [10] we aimed to determine TOF values for essentially full conversion (99%), using optimized times (vide supra). Since TOF values are also dependent on substrate concentration and temperature, we kept the reaction conditions equal. This approach gives reasonably accurate turn-over-frequencies (TOF's) which must be seen as minimal values. Since we assume that the Fe part of the catalyst is fully heterogeneous, only surface atoms can be active. For ideal spherical particles of 5 nm (estimated by p-XRD), the degree of dispersion (ratio of surface to total atoms) is circa 15-20%. This means that, calculated per surface atom, our catalysts are at least a factor 5-6 times more active.
Since the reactors are closed off from the H2 source during the hydrogenation experiment, the given pressure relates to the starting pressure. The pressure drops during H2 consumption but in all cases there was a residual H2 pressure of at least 5 bar after full conversion. At low H2 pressure, the quantity of H2 in the reactor is not sufficient for full substrate conversion. Therefore, runs at low pressure were conducted with reactors in which the gas inlets were kept open to the H2 source. This was only possible for substrates of which the boiling points are substantially higher than the reactor operating temperature.
(2) Catalytic activity of Fe 0 : The catalytic activity of activated Fe 0 without any cocatalyst has been investigated (Table S3). While terminal alkenes and cyclic internal alkenes like cyclohexene can be fully converted, the Fe 0 catalyst is hardly active in reduction of internal linear alkenes (e.g. 3-hexene) and fully inactive in reduction of 1,1-diphenylethylene or tri-substituted alkenes (e.g. 1-phenylcyclohexene and 1methylcyclohexene). As it also failed to reduce benzene, Ba 0 alone is clearly the better hydrogenation catalyst (numbers in Table S3 are taken from ref. [9]). Only for 1-hexene the Fe 0 catalyst was shown to be slightly more active than the Ba 0 catalyst. S17

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(3) Catalytic activity for BaFe at a constant low pressure of 6 bar: A metal mixture of Ba 0 and Fe 0 is also at low constant H2 pressure active in hydrogenation catalysis (Table S4). For runs at higher temperature (120-150 °C) the reactor was pressurized and before heating shut off from the H2 source in order to prevent boil-off of the substrate. This means that there is a natural pressure drop during hydrogenation.
In hydrogenations at lower temperature the valve to the H2 source could be kept open and consequently these were run at a constant H2 pressure. Table S4 shows that alkene and arene hydrogenation with the BaFe catalyst do not necessarily need a high H2 pressure but can also be run at a low constant H2 pressure of 6 bar. Not only the H2 pressure but also the substrate concentration has no effect on the conversion rate: the hydrogenation of benzene to cyclohexane is essentially not affected by dilution with heptane (Table S5).  Cyclohexane could not be detected and the catalyst is unchanged ( Figure S12). However, the addition of 1.5 mol% of MVS-activated Fe 0 led to full conversion to cyclohexane ( Figure S13). The Fe 0 catalyst could be separated from the mother liquor with a magnet. The no-D 1 H NMR spectrum of the mother liquor S22 shows that complex [(BDI)MgH]2 is unchanged ( Figure S14) (Table S7).
Under no circumstances did we find any acceleration for the catalytic activity of Ba 0 by addition of  [11] we used in catalysis experiments the double amount of pyrophoric iron. In all cases, the most active Fe cocatalyst was found to be Fe 0 powder obtained by MVS.   (Table S8). The activity increases linearly with Fe content but reaches its peak at an equimolar ratio (Fig. S15). As additional Fe hardly affects the activity, hydrogenation catalysis was performed with 1 / 1 mixtures.  (Table S9 and S10). It should be noted that Mg 0 alone in the form of a highly pyrophoric activated powder is fully inactive in benzene hydrogenation. In contrast, the MgFe combination shows a significant activity.

Investigations towards the mechanism and the nature of the BaFe catalyst
The catalyst system that consists of Ba 0 and Fe 0 in a 1/1 ratio has been further investigated in order to draw conclusions on its nature. The following observations have been made:

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(1) Grinding of the metals Thorough mixing of Ba 0 and Fe 0 powders with a mortar and pestle increases the activity of the BaFe catalyst by at least a factor of 10 (Table S11). Table S11. Impact of metal grinding on catalyst activation (1 mol% BaFe, 35 bar H2).

(2) Catalyst poisoning with metallic mercury
The activity of the BaFe catalyst is affected by addition of metallic mercury (Table S12). Although controversial, [12] inhibition of catalytic activity by Hg 0 suggests that a part of the catalytic system is heterogeneous.
Table S12. The effect of addition of metallic mercury on the activity of a BaFe catalyst in hydrogenation of C6H6 (750 L, 1.5 mol% BaFe, 1.5 h, 150 °C, 50 bar H2).

(3) Catalyst recycling
The BaFe catalyst has been recycled at least three times and could be reused without loss of activity (Table   S13). Benzene was converted to cyclohexane using a BaFe catalyst. After separation of the spent catalyst with a magnet (Fig. S16), the catalyst was tested again for its activity. This procedure was repeated. The

Substrate Treatment T [°C] t [h] Conv. [%]
Benzene unground 120 1 6 catalyst remained active and did not show signs of activity loss. The mother liquor after a catalytic run is colorless and is not paramagnetic, excluding significant formation of organo-iron complexes. There is no residue after removal of all volatiles indicating that the catalyst after the catalytic run is insoluble. Also after using a larger quantity of BaFe catalyst (1.0 g) no visible quantities (naked eye or microscope) of catalyst remain after evaporation of all volatiles from the mother liquor. Proof for the presence of reducing Ba 0 metal -Reaction with benzophenone: Addition of benzophenone to a spent BaFe catalyst suspended in a toluene/THF mixture led to immediate formation of a dark-blue color, indicative for formation of the radical anion Ph2CO •ˉ (Fig. S17). days. The mixture was then filtered and analyzed by 1 H NMR (Fig. S20). Although the expected product Me3SiH has a boiling point of 6 °C, it could be clearly detected in the mother liquor (Si-H signal at 3.99 ppm, multiplet). Also a sharp singlet at 3.20 ppm was observed which could be assigned to the distribution product SiH4. [13] S33 Figure S20. 1 H NMR spectrum of the gaseous product that has been formed from a used BaFe catalyst (used in the C6D6 to cyclohexane-d6 reduction) which was quenched with Me3SiCl.

Monitoring of hydrogenation catalysis by 1 H NMR
The conversion in catalytic hydrogenation experiments with the BaFe catalyst was determined by 1 H NMR and GC-MS measurements. These spectra have been ordered according to substrate in the same order as in Fig. 3 in the manuscript. Also reference spectra for the substrates and for catalytic runs with only Fe 0 or only Ba 0 (additional data to those reported in ref. [9]) are incorporated.        Figure S36. 1 Figure S45. 1 H NMR reference spectrum (600 MHz, CDCl3, 298 K) of toluene. S47 Figure S46. 1 Figure S48. 1 H NMR reference spectrum (600 MHz, CDCl3, 298 K) of para-xylene. Figure S49. 1 Figure S56. 1 H NMR spectrum (600 MHz, CDCl3, 298 K) after catalytic hydrogenation of naphtalene (100 mg, 0.78 mmol in 750 µL n-heptane, 50 bar H2) with 10 mol% BaFe at 150°C for 24 h. As a main product (>99%) conversion to decahydronaphtalene was confirmed by GC/MS and NMR analysis. Figure S57. 1 Figure S71. 1 H NMR reference spectrum (600 MHz, CDCl3, 298 K) of N-benzylidene-tert-butylimine.